Atoms are the fundamental building blocks of all matter. The possibility of rearranging these particles depends on which part of the atom is being changed and the energy required. Atomic rearrangement spans a spectrum of phenomena, from common chemical changes, which affect how atoms connect, to extreme nuclear events, which alter the identity of the atom itself. These transformations are governed by physical laws that dictate both the possibility and the cost of such changes.
Rearranging Atoms Through Chemical Bonds
The most common form of atomic rearrangement occurs during chemical reactions, where atoms join or separate to form new substances. This type of change exclusively involves the valence electrons in the outermost shell. The nucleus, which defines the element’s identity, remains completely untouched during chemical rearrangement. Atoms participate in this process to achieve a more stable, lower-energy state.
Chemical bonds, the forces holding atoms together in molecules, are formed either through the sharing of electrons (covalent bonds) or the transfer of electrons (ionic bonds). Covalent bonds form when atoms share electrons, such as when hydrogen and oxygen atoms join to create a water molecule. Ionic bonds result from the complete transfer of one or more electrons, creating ions with opposite charges that are then attracted to each other, like the sodium and chloride ions that form table salt. Reactions require an initial energy input, called activation energy, to first break the existing bonds in the starting materials, known as reactants.
Once the initial bonds are broken, the atoms can rearrange their connections to form new product molecules. For instance, in the combustion of methane gas, the carbon, hydrogen, and oxygen atoms rearrange to form carbon dioxide and water vapor. This rearrangement of electron connections is a relatively low-energy process. This allows chemical reactions to be pervasive in nature and easily initiated in a laboratory setting.
Structural Variations of the Same Atoms
A specific type of rearrangement occurs when the identity and count of the atoms remain the same, but their spatial organization is altered, resulting in completely different properties. This structural change is categorized into two main groups: isomerism and allotropy.
Isomerism
Isomerism applies to compounds where two molecules share the exact same chemical formula but have their atoms connected or positioned differently in three-dimensional space. For example, glucose and fructose are both simple sugars with the formula \(\text{C}_6\text{H}_{12}\text{O}_6\). The varied arrangement of their atoms gives them distinct properties.
Allotropy
Allotropy describes elements that can exist in multiple structural forms in the same physical state. Carbon is a widely known example, arranging its atoms in various ways to create materials with vastly different characteristics. When carbon atoms are arranged in a three-dimensional tetrahedral lattice, they form diamond, known for its extreme hardness. Alternatively, when carbon atoms arrange in flat, hexagonal sheets, they form graphite, which is soft and and used as a lubricant. Oxygen also exhibits allotropy, existing most commonly as \(\text{O}_2\) (dioxygen) but also as \(\text{O}_3\) (ozone). Ozone possesses a much stronger oxidizing power than dioxygen.
Transmutation and Nuclear Rearrangement
Transmutation is the form of atomic rearrangement that alters the contents of the atom’s nucleus to change one element into another. Since the number of protons in the nucleus defines the element, transmutation requires a change in this proton count. This process is fundamentally different from chemical reactions and demands immense amounts of energy to overcome the powerful nuclear forces binding the nucleus together.
Fission and Fusion
One method of nuclear rearrangement is fission, where a heavy atomic nucleus, such as uranium-235, is split into two or more smaller nuclei, releasing a large amount of energy and several neutrons. Conversely, fusion involves combining two light atomic nuclei to form a heavier one. Fusion is the process that powers the sun and stars.
Decay and Artificial Transmutation
Nuclear rearrangement can also occur spontaneously through radioactive decay, where unstable nuclei naturally transform into a more stable configuration over time. This natural process, like the decay of potassium-40 into argon-40, is a form of elemental change that requires no external energy input. Artificial transmutation is achieved by bombarding stable elements with high-energy particles in particle accelerators. This converts one element into another, often creating new, heavier transuranium elements.
The Fundamental Laws Governing Atomic Change
All forms of atomic rearrangement, whether chemical or nuclear, are governed by foundational physical principles.
Conservation of Energy
The first principle is the law of conservation of energy, which dictates that energy cannot be created or destroyed, only transformed from one form to another. In chemical reactions, the energy released or absorbed is a transformation of the chemical energy stored in the electron bonds. In nuclear reactions, a small amount of mass is converted directly into energy, but the total energy of the system remains constant.
The Second Law of Thermodynamics
The second law is the Second Law of Thermodynamics, which introduces the concept of entropy, or the tendency toward increasing disorder in an isolated system. Rearrangement processes that result in a more ordered state, such as synthesizing a complex molecule from simple components, require a constant input of energy to counteract this natural drive toward disorder. Chemical reactions typically proceed spontaneously only if they increase the overall entropy of the universe. Therefore, any controlled attempt to build complex atomic structures must expend energy to create order from a less ordered starting state.